268 Prof. E. Wartmann's third Memoir on Indtiction. 



§ XI. On a new instance of electro-magnetic rotation. 



99. I have had occasion to observe, in the preceding expe- 

 riments, a phaenomenon of rotation which merits to be noticed, 

 not only as a new confirmation of the theory of Ampere, but 

 also on account of the relations which it may have with the 

 facts which will be stated in the following paragraph. 



100. If, after having caused two soft iron cylinders to adhere 

 to the poles of a horseshoe magnet, these cylinders are im- 

 mersed in a solution of sulphate of copper contained in a vessel 

 of any form and protected from vibration, we perceive, at the 

 end of a few minutes, when the deposit of reduced copper is 

 already quite visible, a double current established in the mass 

 of the liquid. This current often affects much more the lower 

 than the upper molecules; sometimes, according to the den- 

 sity of the fluid and the state of its surface, it is only established 

 in its interior. In order to be able to examine it well, the 

 liquid mass must be illuminated, either directly or by means 

 of mirrors or refractors. A convenient arrangement consists 

 in placing this mass in a glass vessel with smooth sides, and 

 illuminating it in the dark chamber by a bundle of solar light. 

 The tint of this bundle may be varied from red to violet, by 

 interposing in its course coloured media, or by refracting it by 

 a prism, without the phoenomenon being modified. The 

 movement is rendered perceptible from the displacement of 

 the particles suspended in the liquid ; if the latter is perfectly 

 limpid, it is not observed. It is easily produced by employing 

 a concentrated solution of sulphate of copper, from which 

 some small particles of oxide have been precipitated by a few 

 drops of potassa; these particles remain in suspension. This 

 current ceases at the end of a longer or shorter time, with the 

 chemical action which gives rise to it, when the quantities of 

 copper deposited and of iron dissolved have reached a certain 

 relation. It does not occur in solutions of subacetate of lead, 

 of iron-alum, of triple sulphate of copper, zinc and iron, what- 

 ever pains are taken to establish in their interior precipitates 

 of oxide, or to add to them light powders. Sometimes it lasts 

 for more than two hours. Pure water, rendered slightly opa- 

 line by particles of oxide of copper, presents no movement. 



101. The current is never pi'oduced except under the influ- 

 ence of the magnet. Soft iron cylinders immersed in sulphate 

 of copper are of themselves incapable of producing it; they 

 only accjuire this power when magnetism is induced in them. 

 If an electro-magnet be employed, the rotation varies with the 

 direction of the electric current. It takes place circularly or 

 elliptically around each magnetic pole, and is directed like the 



